So-called total endoprostheses are known for different diseases and injuries of a hip joint. These total endoprostheses consist of two portions, namely, of a shell which (usually after previous cutting) is to be cemented in the hip pan and usually consists of plastic material which is compatible with tissue, and of an elongate, relatively large, prosthesis portion which consists of a suitable steel alloy. On one end of this prosthesis portion, there is arranged a sphere or ball which replaces the femur head of the injured bone. The sphere is connected through a passage part and a flange to a curved shaft which, after previous separation of the head of the femur neck, is inserted in the bone and is anchored there with bone cement.
Compared to the previously known possibilities for treatment of such fractures and diseases, like arthrosis and the like, very good results have been achieved by means of these known total endoprostheses. It cannot be denied, however, that the rate of complications is great enough so that improvements are necessary.
Consideration must be given to the fact that the operation of inserting such a total endoprosthesis in the respective body is, in almost all respects, very serious. This is so because bone parts of the hip joint of a considerable extent have to be removed which, for example, in the case of wear phenomena, are not injured per se and would be suitable for the intended supporting function.
Apart from fractures of the femur neck which obviously represent special circumstances and which frequently require such an operation (especially in so-called long fractures), there still remains great fields of use for diseases which basically do not require such a radical operation.
Therefore, such radical operations are no longer considered in such hip joint diseases, and it has been tried to replace only the sliding areas, which are especially important for the operability of the hip joint and are simultaneously highly stressed. This can be accomplished by cementing a shell-shaped pan of suitable plastic material or of ceramics into the hip joint pan, similar to the previously described total endoprosthesis. Also included in the procedure is the replacing of the counter-sliding area for the artificial hip joint pan by a shell which consists of metal, namely, of suitable steel, which has on its outer side the shape of a spherical segment. The height of the spherical segment usually is a little smaller than the equatorial height, i.e, smaller than the radius of curvature of the outer side of the spherical segment.
In the interior, the known metal shell for the femur head has a cylindrical bore which, on the end face, leads into a spherical surface section.
The curvature of this spherical area section on the inner side of the known femur head shell corresponds to a curvature on the femur head after a respective cutting of the end face of the femur head. The diameter of the cylindrical internal wall of the known femur head shell corresponds to the diameter of the femur head, after respective cutting.
Such a hip joint prosthesis, also known as a shell prosthesis, has obviously the advantage, in comparison with the above-described total endoprosthesis, that considerably less bone tissue has to be removed and, therefore, considerably less foreign material has to be inserted into the body.
It turns out in practice, however, that these theoretical advantages are again compensated to a considerable extent by disadvantages of these known shell prostheses.
First of all, no precise fitting surfaces can be produced during the cutting of the cylindrical and spherical end sections of the bone, because the freely guided cutting instrumentation does not have such qualities and because this instrumentation removes tissue material just where the smallest working resistance occurs. This more or less free working, however, not only leads to imprecise fitting areas but it is impossible to construct the implant physiologically in axis alignment.
Furthermore, the inner edge of the metal shell which is applied fittingly on the previously treated bone exerts, in the case of subsequent static and dynamical loads, a groove or notch effect on the bone which leads to groove fracture some time afterwards.
Moreover, a disadvantage in this known hip joint prosthesis is that the metal shell, also like the hip pan shell, is anchored with cement and, in fact, usually with a significant amount of cement which leads again to an insufficient amount of the tissue of the femur neck head. In the region of the bone shaft, this often leads to a spongiosis of the bone with subsequent disintegration which, in turn, results in incorrect positioning of the supporting surfaces or errors in the bone removal processes. However, both phenomena are, of course, extremely disadvantageous because the bradytrophic tissue of the cartilage has to absorb and transfer, in addition to its function of movement, the occurring forces and moments.